1. Field of the Invention
The present invention relates to a novel magnetic recording medium.
2. Description of Prior Arts
A magnetic recording medium basically comprises a nonmagnetic support and a magnetic recording layer provided on the support, and the magnetic recording layer generally comprises a binder and a ferromagnetic powder dispersed therein.
Recently, a demand for a higher density recording system has increased, and hence a ferromagnetic metal powder has been employed in place of a conventional metal oxide-type ferromagnetic powder such as .gamma.--Fe.sub.2 O.sub.3 and iron oxide-type ferromagnetic powder containing other component such as cobalt.
The ferromagnetic metal powder is known to afford recording of a higher density, as the powder size is more minimized.
As a binder for dispersing the ferromagnetic metal powder therein to form a magnetic recording layer, there has been widely employed a resin component comprising both of a vinyl chloride copolymer (e.g., a vinyl chloride/vinyl acetate/maleic anhydride copolymer) and a polyurethane resin in consideration of low hardness of the ferromagnetic metal powder. Employment of such combination as a binder affords to produce a magnetic recording layer highly improved in various properties (e.g., strength) required for a magnetic recording medium.
However, according to study of the present inventors, a ferromagnetic powder is poorly dispersed in the above-mentioned binder particularly in the case of using a minimized ferromagnetic powder. That is, in spite of using such minimized ferromagnetic powder, the resulting magnetic recording medium is not improved sufficiently in electromagnetic conversion characteristics, and some particles of the poorly dispersed ferromagnetic powder are easily dropped off from the recording medium in the running procedure. A portion of the particles dropping from the medium in the running procedure is apt to instantaneously adhere to a magnetic head, and in that instant, lowering of reproduction output (so-called instantaneous clogging on the head) occurs.
For enhancing the dispersibility of a ferromagnetic powder, it is known to incorporate a polar group into the binder so as to improve an affinity of the binder for the ferromagnetic powder.
For instance, Japanese patent publication No. 59(1984)-8127 discloses an invention relating to a magnetic recording medium using at least one of a vinyl chloride copolymer having a specific polar group and a polyurethane resin in combination with a polyurethane resin having a specific polar group as a binder for the purpose of enhancing the dispersibility of the ferromagnetic powder.
Although the employment of such a binder improves the dispersibility of a ferromagnetic powder, there is also brought about a problem, that is, the surface of the magnetic recording layer is highly smoothened owing to the well dispersibility of the ferromagnetic powder so as to excessively increase the friction coefficient of the resulting magnetic layer. Accordingly, it is required to lower the friction coefficient of such smooth surface of the magnetic recording layer for providing an excellent running property to the resulting magnetic recording medium. The above-mentioned publication also discloses an example of using a lubricant such as a silicone oil, but it has been confirmed by the present inventors that the effect by using such lubricant is not satisfactory.